As far as making mistakes in class and pointing them out---as a rule I never single out people who are making mistakes and then talk about them. I don't want to make someone feel like they're "not so good" while others are good. As an instr, I would suggest to let them practise and YOU make mental notes. When it's time to go to the next technique, take what ever needed time and talk about the mistakes and demonstrate the correct without naming names. They will know who you are talking about without feeling like they are being picked on. Now, if you're showing mai otoshi and someone is doing shiho nage, then you have to say something, obviously. But if a foot isn't properly placed etc, just make a mental note and go over it later. I always reserve, after a 1 1/2 hour class, an extra 1/2 hour for anyone who might want to ask a question and needs extra help in understanding, one on one. That works out really well and they do appreciate it. It shows that, as an instr, you care and the students welcome that attitude from an instr.